Chris Kattan -- Arrested for DUI

Chris Kattan was arrested for DUI early this morning after his car struck a vehicle ... and TMZ has learned he allegedly confessed he was on prescription meds.

Law enforcement sources tell TMZ ... the former "Saturday Night Live" cast member was arrested on the 101 Freeway in the San Fernando Valley just after 1 AM.

Another driver called CHP to report another driver in a Mercedes who was "weaving all over the roadway at slow speeds."

Kattan allegedly then crossed into an area that was coned off for a freeway closure and he struck a Dept. of Transportation vehicle from behind.

Kattan was not injured and apparently there was no one in the DOT vehicle.

Cops say Kattan appeared out of it. Our sources tell us he was arrested and booked for DUI ... specifically drugs, NOT alcohol.